The present invention relates to a balanced (differential) power amplifier and a high-frequency communication apparatus having the power amplifier.
One of the major issues in portable wireless communication terminals such as cell phones and PDAs (Personal Digital Assistances) with wireless LAN function is to develop power amplifiers with improved efficiency and reduced power consumption.
For example, as stated in detail in JP 2001-94360 A, the power amplifiers are circuit components in which lower power of several milli-Watts order produced in a signal processing section in a portable terminal is amplified at once up to high power of almost Watt order and transmitted to a transmission antenna. Thus, the power amplifiers process signals of high power, and therefore their power consumption is several orders of magnitude larger than that of most of other electric components, which largely affects battery consumption.
Approaches to develop power amplifiers with improved efficiency and reduced power consumption include one using a distortion compensation circuit stated in the aforementioned JP 2001-94360 A. In this approach, the distortion compensation circuit is used to suppress the distortion component of an output signal from a power amplifier so that the power amplifier operates in the vicinity of the region of higher nonlinearity (the maximum saturated power region or the class-C bias region) to improve efficiency and reduce power consumption.
FIG. 12 is a circuit diagram showing a power amplifier having the distortion compensation circuit disclosed in JP 2001-94360 A. As shown in FIG. 12, a grounded-emitter bipolar transistor 1004 amplifies high-frequency electric signals inputted from an input terminal 1001 and outputs them to an output terminal 1002. Moreover, direct-current bias applied to the base of the transistor 1004 is fed from a voltage source 1003 via a variable impedance element 1005. Moreover, an impedance regulator circuit 1008 is connected to a midpoint between the voltage source 1003 and the variable impedance component 1005.
The impedance regulator circuit 1008 is structured such that a series circuit composed of a resistance 1006 and a capacitance component 1007 is short-circuited to the ground.
Generally, most power amplifiers are designed to have a single-end structure, though some power amplifiers are designed to have a balanced structure for specific purposes. For example, JP 2001-267857 A discloses an example in which a power amplifier of balanced structure in high-frequency bands is implemented for the purpose of high-efficiency push-pull amplification.
FIG. 13 is a schematic view showing the configuration of a balanced amplifier shown in the aforementioned JP 2001-267857 A. In this balanced amplifier, a high-frequency electric signal inputted from an input terminal 1401 is divided by a reversed-phase power divider 1405 into signals whose phases are different by 180 degrees from each other. These reversed-phase signals are amplified by two unit amplification circuits 1403, 1404, and then combined efficiently by a reversed-phase power combiner 1406 before being outputted from an output terminal 1402.
FIG. 14 is a schematic block diagram showing a transmission system in a conventional high-frequency communication apparatus, more precisely, a schematic block diagram showing a transmission system of a multi-mode and multi-band wireless communication apparatus supporting a plurality of different communication systems, which is demanded from the fourth-generation cell phones (System Beyond IMT-2000) and the like.
As shown in FIG. 14, the high-frequency communication apparatus is composed of four communication systems, a 800 MHz-band cell phone, a 1.9 GHz-band cell phone, a 2.4 GHz-band wireless LAN and a 5.2 GHz-band wireless LAN.
In the high-frequency communication apparatus, inside a casing 1321, a transmission signal in the 800 MHz-band produced by a signal source 1313 is amplified by an amplifier 1305 and radiated from an antenna 1301, while a transmission signal in the 1.9 GHz-band produced by a signal source 1314 is amplified by an amplifier 1306 and radiated from an antenna 1302. Further, a transmission signal in the 2.4 GHz-band produced by a signal source 1315 is amplified by an amplifier 1307 and radiated from an antenna 1303, while a transmission signal in the 5.2 GHz-band produced by a signal source 1316 is amplified by an amplifier 1308 and radiated from an antenna 1304.
These four signal sources 1313 to 1316 are generally implemented by RFIC (Radio Frequency Integrated Circuit) technology and their output lines 1317 to 1320 are generally differential lines, whereas four amplifier circuits 1305 to 1308 are generally of single-end structure. Because of this reason, four baluns 1309 to 1312 are inserted for connecting these signal sources and amplifier circuits.
However, the power amplifier with the distortion compensation circuit shown in FIG. 12 is produced on the assumption that the amplifier circuit is of single-end structure. Consequently, in the case where, for example, two unit amplifier circuits are disposed in parallel to form a balanced amplifier circuit, applying the distortion compensation circuit shown in FIG. 12 to the balanced amplifier circuits doubles the number of component parts of the distortion compensation circuit section, thereby posing a problem that the amplifier circuit is upsized.
Moreover, the balanced amplifier circuit receives the input of two types of high-frequency electric signal, an odd mode (reversed-phase component) signal and an even mode (common-mode component) signal. Therefore, in installing the distortion compensation circuit in the balanced amplifier circuit, the circuit operation of the distortion compensation circuit is not uniquely determined and should be of various types depending on uses, which causes a problem that respective uses requires solutions.
For example, in the balanced amplifier shown in FIG. 13, on a line shown by a reference symbol M in FIG. 13, an even mode (common-phase component) that is an unnecessary parasitic component is present in addition to an odd mode (reversed-phase component) that is an originally intended electric signal. Consequently, in installing the distortion compensation circuit in the unit amplifiers 1403, 1404, the distortion compensation circuit should be operated normally for the odd mode, whereas the operation of the distortion compensation circuit should be changed for the even mode depending on uses. More specifically, for the even mode, the operation needs to be changed corresponding to the cases where the distortion compensation circuit should be operated, where the distortion compensation circuit should not be operated, and where the circuit should be operated not as a distortion compensation circuit but as an attenuator. For these three cases, different circuit operations are necessary.